In a closed air conditioning or refrigeration system there are a number of methods of unloading that can be employed. Commonly assigned U.S. Pat. No. 4,938,666 discloses unloading one cylinder of a bank by gas bypass and unloading an entire bank by suction cutoff. Commonly assigned U.S. Pat. No. 4,938,029 discloses the unloading of an entire stage of a compressor and the use of an economizer. Commonly assigned U.S. Pat. No. 4,878,818 discloses the use of a valved common port to provide communication with suction for unloading or with discharge for Vi control, where Vi is the discharge pressure to suction pressure ratio. In employing these various methods, the valve structure is normally fully open, fully closed, or the degree of valve opening is modulated so as to remain at a certain fixed position. One problem associated with these arrangements is that capacity can only be controlled in steps or expensive motor driven modulation valves must be employed to fix the valve opening at a certain position for capacity control.